A thermostat is a conventional device utilized to control the output of heating and cooling systems. The same basic thermostat may be used to control a variety of different designs of heating and cooling systems. The thermostat is typically part of a control loop that is used to match the current capacity operating of the heating and refrigeration system to the heating or cooling load presented by the zone being heated or cooled. As used herein, the term air conditioning system will refer to both heating and cooling systems.
Typically, a sensor is located within the zone being air conditioned. The sensor senses the temperature of the zone and provides a signal representative of the sensed temperature to the thermostat. In the past, the sensor was frequently incorporated within the body of the thermostat. More recently, the sensor has been used as a zone sensor providing a signal to a thermostatic controller remote from the zone under control. For purposes of this application, both shall be referred to as a thermostat.
The thermostat additionally is in receipt of an external command known as a setpoint. The setpoint is the temperature desired to be maintained within the space. The setpoint is set manually by the user adjusting the temperature dial on the thermostat. Alternatively, many thermostats currently available incorporate a microprocessor that has the capability to automatically change the setpoint on the occurrence of some event. In the heating season, the setpoint is usually automatically changed with the time of day to have a lower setpoint at night and a more elevated setpoint during the day. The opposite may be the case during the cooling season.
The thermostat compares the temperature information received from the sensor with the setpoint and generates an error signal. The error signal is provided to a control system within the air conditioning system. This control system functions to respond to the error signal by increasing or decreasing the temperature within the space. The control system may comprise valves, dampers, electric relays, and electric motor speed controllers, and may control a number of different pieces of air conditioning equipment in order to effect the temperature change that is commanded by the error signal. Such equipment may include a heat exchanging coil, fan, steam generator, or other means of heat exchange.
There are typically two modes of control of an air conditioning system, the steady state mode and the transition mode. The steady state mode controls the air conditioning system about a setpoint, which is the temperature setting of the thermostat. This setting may either be set manually by the user or it may be set in response to an automatic programmed setting selected by a microprocessor associated with the thermostat. The transition mode controls the air conditioning system when the system is in transition from one setpoint to a newly selected setpoint.
The transition mode is entered into in response to an increased or decreased setpoint. In the transition mode, the air conditioning system will turn full "on". The air conditioning system will stay full "on" until the sensor in the zone sends a signal to the thermostat indicating that the temperature in the space is equal to that of the setpoint. At that time, the air conditioning system reverts to its steady state mode of control related to the setpoint.
A problem arises during setpoint changes because the temperature sensor in the zone has a certain lag in it. Typically, the sensor is located on a wall. Air movement across the surface of a wall does not readily occur due to friction of the air with the wall. This can result in a layer of dead air immediately adjacent to the wall. Moreover, the wall may actually act as a heat sink, drawing heat from the air next to the wall and thereby affecting the temperature of the air that is being sensed by the sensor. Further, it takes a certain time for the heated or cooled air entering the volume of the zone to circulate to the wall surface in the vicinity of the thermostat. All these factors combine to create a significant lag in the time it takes for the sensor that is associated with the wall-mounted thermostat to report the actual temperature in the zone.
The sensor is accordingly always sending what amounts to a delayed temperature signal to the thermostat. The delayed temperature is representative of what the actual temperature in the zone was at some previous time. In the transition mode of operation, this results in the air conditioning system being kept on for a substantial length of time after the actual temperature in the zone exceeds the setpoint temperature. In heating systems, the resultant overshoot in temperature is undesirable because it is wasteful with respect to energy usage and it is uncomfortable for the occupants of the space. It may take up to several hours for the zone to stabilize at the setpoint temperature after an overshoot occurrence. The same phenomenon occurs during cooling of the zone when the setpoint is reduced. In this case the temperature in the zone considerably undershoots the setpoint temperature.
Such setpoint changes are common with present clock thermostats having microprocessors contained within them as temperature setpoints are sought that will maximize the energy efficiency of the air conditioning system. These thermostats can command many set up and set back cycles for energy conservation in a single day. Additionally, there are many air conditioning system installations in which the capacity of the air conditioner greatly exceeds the load needed to heat or cool the space. In these systems, making a major setpoint change causes the over capacity system to go to the full "on" condition. These systems experience serious overshoot because of the over capacity of the system. Furthermore, some systems with an automatic "unoccupied" mode allow an after hours occupant to override the unoccupied setpoint by activating a manual switch. After a period of time, the unoccupied mode will be reset automatically. Accordingly, there is a need for thermostats to efficiently control the transition from one setpoint to another with an absolute minimum of overshoot or undershoot.
A number of methods have been attempted in order to control temperature overshoot and undershoot. An example is the method put forth in U.S. Pat. No. 4,615,380. This method involves monitoring each overshoot for a period of time after an increase in the setpoint temperature has been commanded. When an overshoot that exceeds a predetermined value is detected, a gain in the system is changed in order to attempt to reduce the overshoot on the next setpoint temperature increase command. This method requires circuitry to monitor and then store data on all overshoots occurring over a period of days, as well as to set a gain based on an overshoot exceeding some predetermined value. It also appears that this system is based on the setpoint change always being the same number of degrees and is designed to function with automatically controlled thermostats.
It would be a decided advantage to be able to avoid overshoot and undershoot under all operating conditions. The means of eliminating overshoot and undershoot should be simple and inexpensive. The method should be adaptive to accommodate all changes in setpoint, not just those of a fixed number of degrees. It should effectively make use of the sensed temperature in the zone, realizing that the sensed temperature significantly lags the actual temperature in the zone.